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Cover crops: How are they best used?

Insights05 Sep 2016by Abby Kellett

With some farmers sceptical about the viability of cover crops, the Allerton project hosted an event which focused on the benefits they can offer and provided advice on how they can be managed for best results.

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The event, supported by Kings Crops and Dale Drills, included talks from farmers and scientists who have delved into the practicality of crop management and associated costs.

One thing all the speakers agreed on was there were numerous benefits to be gained from having a growing crop in the ground throughout the year.

Benefits and challenges of cover cropping

Benefits

Increases soil organic matter

Can remove compaction

Improves infiltration/reduces run-off

Reduces wind/soil erosion

Can aid weed control

Improves soil structure

Increases soil biology

Increases nutrient retention

Challenges

Cost of seed, establishment and destruction

Reduced benefits where cover crop are cultivated

Reliance on glyphosate for destruction

Potential for ‘green bridge’ effect

Short window for establishment due to weather conditions in some regions

Allerton Project Head of Research, Chris Stoate, suggested improved soil organic matter was one of the primary advantages of cover crops.

He said: “We should be aiming for five to six per cent soil organic matter. This is where we get the best crop performance in terms of rooting capacity, nutrient cycling, biological activity, infiltration of water and reduced run-off.”

Experiments that Mr Stoate has been involved in indicated that between 0.3 and 0.6 tonnes of sediment per hectare per year were lost to water courses, something increased soil organic matter could help mitigate.

As well as sediment deposition, chemicals from agriculture were compromising the quality of water supplies, making product removal all the more likely, according to Mr Stoate.

“We are currently seeing a peak in propyzamide in water courses later on in the year that far exceed the 0.1mg threshold.

“We need to take this seriously as without propyzamide, we have serious problems in controlling weeds like black-grass,” he said.

“By getting a cover crop in, we are capturing sunlight and carbon and are putting roots into the ground. Those roots are very capable of removing compaction,” he said.

In sloping fields where he uses shallow cultivation, he uses cover crops to keep soil in place, which he believes avoids the need for variable rate technology.

“Cultivation along with heavy rainfall causes soil particles to move down slopes and so we would have to start managing the field with variable rate seeding and variable P and K to accommodate varying soil depths.

“If we can keep the soil covered by forming a layer of ‘soil armoury’ then our soil will be protected from rainfall and soil movement,” he added “as well as protecting the soil, we can trap lots nutrients.

“When 300kg of N is applied to a milling wheat crop, it uses about 200kg, but where has the other 100kg gone? A lot of it will have moved down the soil profile, but cover crops have the potential to retain this,” he said.

Effect of cultivations

NIAB trials which looked at the interaction between cover crops and cultivation methods indicated the effect of cover crops is greatest in shallow cultivation systems with regards to wheat yields in subsequent crops.

The organisation’s head of crop research communication, Ron Stobart said: “While shallow cultivation with no cover crop gave, on average, the lowest yield, using cover crops in shallow cultivation systems consistently gave the highest yield and increased gross margins by about £50 per hectare per year consistently.

“However, in plough based systems, there was no clear advantage to using a cover crop.”

He urged growers not to expect to see the benefits of cover crops in the following cash crop, but more likely, benefits will be noticed in the crops following that.

Recognising this, Mr Freestone spreads the cost of growing cover crops over six years, since the benefits tend to be long-term.

He said: “The cost gets spread over the length of the rotation because we are capturing nutrients now that won’t be available until following crops. Growers should not be doing this for an immediate return.”

Establishment

While it is tempting to cut costs when establishing a cover crop, Mr Freestone recommended treating it as a ‘proper’ crop.

“In the past we have put cover crops in ahead of the combine by putting a spinner on the back of the sprayer and broadcasting seeds into the standing crop.

“However, too often these crops did not establish well enough. We can’t afford to have bare patches because we have spent money on seed without gaining the benefit.

“Now, we drill all of our cover crops and although there is extra expense, we are getting much more reliable crops.”

Where possible, both Mr Freestone and Mr Stobart suggested sowing cover crops as early as possible.

NIAB research carried out across a number of sites concluded that, when all other factors where ignored, early sowing had clear benefits in the size of cover crop biomass.

Mr Stobart said: “When assessing cover crops in October, those sown in mid-August were large, those sown at the end of August grew fairly well but by September, the size of crops sown was starting to plateau.”

Wanting to make the most of this early season growth, Mr Freestone aims to operate a ‘five minute fallow’, whereby the drill sows the cover crop directly behind the combine.

Mr Freestone said: “If we can get cover crops in following the combine, we have warm soil temperatures, adequate soil moisture, long day length and hopefully some sunshine.

“Then these crops can have a really explosive start in order to create lots of organic matter and lots of roots.”

When assessing the use of starter fertiliser, NIAB found that an initial dose of around 30kg N increased cover crop biomass by about 15 per cent, not a huge increase according to Mr Stobart.

Furthermore, there was no increase in plant populations but a significant increase in weed numbers as a result of early nitrogen application.

“In some cases, we saw a doubling in weed numbers, a lot of that being black-grass.

“This could be a good thing if farmers a looking to draw weeds out of the seedbank in order to spray them off with glyphosate,” said Mr Stobart.

In other research, applying starter N actually reduced the below ground biomass. Mr Stobart said this was likely to be due to consequent ‘lazy rooting’.

“If roots are supplied with adequate amounts of N on the soil surface, why would they go elsewhere?

“Actually, if we want to get big long roots that explore the soil profile and open up the soil, we need to make roots work for nitrogen a bit.”

Variety choice

Speakers agreed that each species in a cover crop mix should be selected for a particular job, whether it be removing soil compaction, trapping nitrogen or creating tilth. By focusing on one or two aims, they said targets are more likely to be met.

However Mr Stobart added that using multiple species in a mix can reduce the risk of the crop failure. “I think mixes add utility, if one doesn’t do well, the chances are something else will.”

“But I am not a big believer that you need 20 cover crops in a mixture.”

In trials conducted by NIAB, Mr Stobart found that legume species were very effective in improving subsequent cash crop yields, when sowing was not delayed.

“Some of my best results came from legume species. But unless they are being planted in the middle of august, you probably won’t get sufficient growth from them.”

Mr Freestone has also seen benefits from using legume species, when sown in August, in particular when using vetch.

“A pure vetch crop sown on August 7 had really good return of organic matter, delivering over 5 tonnes of organic matter per hectare to the soil.

“When we measured the amount of nitrogen within the vetch canopy, it had 285kg of nitrogen. It is fixing free nitrogen that will be available to you at some point in the rotation,” he said.

The deep tap roots associated with brassicas play an important role in creating soil structure, but while they are a familiar species to grow, Mr Stobart warned that care needs to be taken when brassicas, such as oilseed rape, exist in the rotation.

The brassica radish is used on Mr Freestone’s farm to improve drainage on some of his heavier soils. Additionally, the deep roots help break through compaction layers and retrieve nutrients that may otherwise leach.

Where he adopts a no-till system, he has found the extensive side rooting of phacelia plants help create a better seed-bed prior drilling, without the need for cultivation.

“Phacelia, which is good at putting side roots out has been fantastic for creating tilth. This is really important when going in following a cover crop with a zero-till drill.

While farmers at the event appreciated the benefits of growing cover crops, the associated cost seemed a stumbling block for many. However, Mr Freestone assured that it does not have to be expensive.

“Where possible, grow your own cover crops. If you’ve got oats or linseed in your rotation, use some of those to do some home-grown cover cropping.”

To help reduce costs he also suggested charging livestock farmers to have their cattle graze the crop over the winter or have them pay for the seed.

“My aim is to get more livestock on the farm, they help reduce the overall biomass of the cover crop making it easier to handle when it comes to drilling.

“You don’t want to graze the cover crop down completely, if you allow it to be grazed to about half its volume you have enough plant there to intercept rainfall and the protect the soil,” he said.

Take home messages

Aim for five to six per cent soil organic matter

Cover cropping can have the biggest effect on subsequent yield in shallow cultivation systems

Sow cover crops as early as possible to benefit from larger crop biomass